Full Definition of CLOSED

b (1): traced by a moving point that returns to an arbitrary starting point <closed curve>; also: so formed that every plane section is a closed curve <closed surface>(2): characterized by mathematical elements that when subjected to an operation produce only elements of the same set <the set of whole numbers is closed under addition and multiplication>(3): containing all the limit points of every possible subset <a closed interval contains its endpoints>

c: characterized by continuous return and reuse of the working substance <a closed cooling system>